The present invention relates to systems which control equipment, such as heating, ventilation and air conditioning equipment; and more particularly to controlling devices that have discrete states, e.g., on and off, and to control algorithms that pulse-width modulate the operation of the devices.
FIG. 1 conceptually illustrates a typical heating, ventilation and air conditioning (HVAC) system 10 for controlling the environment of a room 12. Air from the room is drawn by fan 15 into a return duct 14 from which some of the air flows through a damper 16 to a supply duct 18. Some of the return air may be exhausted to the outside of the building through an outlet damper 20 and replenished by fresh outside air entering through an inlet damper 22. The dampers are opened and closed by actuators that are operated by a controller 24. Air in the supply duct 16 flows through a direct expansion cooling coil 25 before being fed into the room 12. Temperature sensors 26 and 27 measure the air temperature in the supply duct 18 and the room 12, respectively, and provide signals to the controller 24. A fluid circulates through the cooling coil 25, then flows outside the building to a condenser coil 28 and through a compressor 29 before being returned to the cooling coil.
A heating coil 21 also is provided and selectively receives heated water from a boiler when the room environment needs to be warmed.
Many components of an HVAC system have only two operational states: on and off. Room temperature control based on a thermostat switching a heating or cooling device is an example of such on/off control. Whenever the thermostat indicates that the temperature of the room needs to be adjusted, the only choice is to turn on the heating or cooling device to full capacity regardless of the degree to which the room temperature varies from the desired level. Thus, the device applies the same amount of heating or cooling to the room regardless of the deviation from the desired temperature point, the only difference being how long the heating or cooling device must operate before the desired temperature is achieved. Often, the full operation of the HVAC system provides more heating or cooling than is required which results in the room temperature overshooting the desired value and producing a significant temperature deviation in the opposite direction.
In an attempt to improve control resolution, multiple on/off devices are often grouped together and operated in various combinations and sequences. For example, there may be multiple compressor stages 30, with the number of them which are active at any given time being determined by the cooling load requirements of the room.
A generic problem with controlling on/off devices, such as compressors, whether single or multiple stages, is that loads falling between the maximum and minimum capacities cannot normally be met exactly in steady state. The result is that controllers need to switch on and off continuously in an effort to meet the setpoint. Such cycling on and off taxes the components and can lead to premature failure.
The present invention relates to a method for controlling a device of a facility management system wherein the device controls an operational parameter of a building, such as temperature for example. This control method employs pulse-width, pulse-frequency modulation (PWPFM) so that the duration of the controlled device""s on-time and the cycling frequency are varied in response to the load. This pulse-width, pulse-frequency control strategy is adaptive and has application to devices that are unable to be switched at requisitely high frequencies due to physical limitations and potential equipment wear.
The method senses the regulated operational parameter to provide a measured value. The desired amplitude of the variation in the measured value D is defined by the user. A pulsed output signal h, produced in response to a control signal u, turns the device on and off. The output signal has a cycle period C during which a pulse of a given duration occurs. The cycle period and the given duration are varied in response to the control signal so that the amplitude of variation in the measured value is limited to no greater than the desired value D when the system is in a steady state pulsing condition.
Versions of this control strategy are described for the control of single and multiple stage devices.
The control method adapts to changes in the gain of the controlled process to ensure performance according to the specified value D.